1. Field of the Invention
The present invention relates to a laser light source device that outputs transverse-multimode light, and a laser irradiation apparatus using the laser light source device.
2. Description of the Related Art
In recent years, a laser annealing technique has become popular, in which an amorphous silicon thin film is irradiated with laser light to change the amorphous silicon thin film into a polycrystalline silicon thin film. This process is a modification process contained in laser processes such as modifying, processing (punching, cutting, welding, etc.) a substance, or exposing a substance (lithography). The technique is used for a manufacturing process of a thin-film transistor (TFT) which is applied to, for example, a liquid crystal display device or an organic electroluminescence display device. The technique is also used for a manufacturing process of an optical sensor such as a line sensor, a photovoltaic power generation element such as a high-performance thin film solar cell, or a semiconductor device such as a semiconductor integrated circuit, e.g., a memory LSI, using a semiconductor film. In particular, in a case where a thin film made of polycrystalline silicon or microcrystalline silicon is fabricated by a low-temperature process in which laser annealing is applied to a substrate, electron mobility can be increased as compared with the amorphous silicon thin film while the degree of freedom for the low-temperature process and the substrate structure is assured. Thus, high-speed response is achieved.
For a light-absorptive element, such as a solar cell, light utilization efficiency can be maximized by using the polycrystalline silicon or the microcrystalline silicon. For example, a solar cell is configured to have a tandem structure including an amorphous silicon thin film and a microcrystalline silicon thin film. Hence, the solar cell can efficiently absorb the sun light with a short wavelength, thereby increasing efficiency of the entire cell (see “OYO BUTURI”, a publication of The Japan Society of Applied Physics, Vol. 76, No. 6, 619, 2007).
In addition, when the laser annealing technique is used, microcrystal or polycrystal grains can be fabricated on an inexpensive, large glass or plastic substrate without the substrate heated at a high temperature. Thus, the cost can be decreased and the performance can be increased.
In the laser annealing apparatus, a gas laser, or an excimer laser of pulse light with 100 W or higher in average has been used. Also, a solid laser of a substantially fundamental Gaussian beam, and a relatively small semiconductor laser have been studied and developed. However, to perform excimer laser annealing (ELA), periodic maintenances, such as frequently exchanging gas, frequently replacing a tube, and cleaning an extraction window, have to be carried out. In addition, variation in pulse output may cause crystal grains after annealing to be small and the grain diameter may likely vary. As a result, the mobility of an obtained polycrystalline film becomes small. This is because a variation of a pulse light intensity is large with respect to an average output variation of the laser light, that is, a process margin is small with respect to the average output variation. Also, a silicon thin film has a large absorption coefficient for ultraviolet, and hence, only a portion of the silicon thin film near the surface absorbs the ultraviolet. The silicon thin film becomes polycrystal and a portion of the silicon thin film near a glass substrate becomes silicon microcrystal as the core (see “The Review of Laser Engineering”, a publication of The Laser Society of Japan, Vol. 34, No. 10, 693, 2006).
In contrast, when a solid laser, more particularly, a green laser using second harmonic generation is employed, the periodic maintenances such as those for the gas laser are not necessary. A silicon thin film has different absorption coefficients for different irradiation wavelengths. Thus, the green light with a longer wavelength than that of the ultraviolet reaches a deeper portion of the thin film, thereby heating and melting the entire film of the amorphous silicon. Accordingly, the size of crystal grains becomes large, and the characteristic of the film such as mobility can be improved (see “The Review of Laser Engineering”, a publication of The Laser Society of Japan, Vol. 34, No. 10, 693, 2006).
Further, a configuration has been suggested in which the solid laser is used, and in addition to the second harmonic generation, a fundamental wave oscillated by a laser resonator is partly transmitted through the resonator. Accordingly, a deeper portion of the amorphous silicon thin film is heated with the fundamental wave for which the thin film has a small absorption coefficient, and simultaneously, the amorphous silicon thin film is melted at the surface with a second harmonic wave for which the thin film has a large absorption coefficient (see Japanese Unexamined Patent Application Publication No. 2003-347237).